This invention relates to magnetic separators used for the separation of certain magnetic materials from relatively less magnetic materials and wherein the separator operates on the principle of passing a confined mass of magnetic bodies together with the material to be separated through a magnetic field and flushing out the relatively non-magnetic material by means of a fluid.
Various magnetic separators of the above general type have been proposed and used but the separators at present available do not operate effectively under certain conditions, such as, where a slurry of material is passed through the mass of magnetic bodies and the slurry contains material which tends to be retained by the mass, with the possibility of an ultimate blockage being formed. This problem arises particularly in the case of wet high intensity magnetic separation.
Wet high intensity magnetic separation (WHIMS) is a method used for separating weak magnetic (paramagnetic) material from non-magnetic material. In order to effect separation, high magnetic field strengths and gradients are necessary. The magnetic field is produced electrically either by windings around an iron yoke or by using the field produced inside a solenoid. Gradients are induced inside the separation volume by placing pieces of magnetically soft iron in the field; these distort the lines of force and hence set up high gradients.
Separation of the magnetic and non-magnetic material takes place within the high intensity high gradient separation zone (or volume). The pieces of magnetically soft iron mentioned above are held within a cannister (for a batch machines) or within a rotating annular housing (carousel) which continually moves through the separation zone. This magnetically soft iron is called the matrix. It serves to produce gradients, as mentioned above, as well as to induce higher magnetic fields. The matrix also serves to slow down the flow of pulp in the separation volume, affording the magnetics more opportunity to be influenced by the magnetic forces and thereby improving the separation. The magnetic material is held by the matrix while the non-magnetic material is flushed through the working volume by the flow of pulp and rinse water or other liquid. The magnetic material is then removed by turning off the field (for a batch machine) or by moving the matrix out of the field (for a continuous machine) and washing with water. Clearly, a continuous machine is desirable for industrial application. Usually such continuous machines utilise a matrix which is carried in an annular cross-sectioned housing rotatable avout a generally vertical axis.
The design of the matrix is quite critical to the efficient separation of any given paramagnetic material from non-magnetic material; e.g for the iron ore haematite, which is a relatively strong paramagnetic material, an open type of matrix has been found to be adequate. By open matrix is meant a matrix with a relatively small resistance to flow. However, for the separation of weak paramagnetic materials, such as the mineral suite containing the Witwatersrand gold and uranium values, a matrix with a higher resistance to flow (i.e a more closed matrix) has been found necessary to give efficient performance of these machines.
One of the most effective materials known at the present time for a closed type of matrix is iron balls (spheres). These may range in diameter from about 2 mm to 15 mm or more depending on the duty. Other materials which have been used are shot, rods, woven wire, wedge wire, nails etc.
The major problem that arises with the closed type of matrix is blockage by ferromagnetics and wood fibre. These latter materials are fairly universal contaminants, especially in ore pulps and accumulate in the the matrix with time. When blockage occurs, it is necessary to stop the machine and clean the matrix. This down-time is a serious drawback to the large scale implementation of WHIMS. The use of complicated feed preparation equipment can alleviate the problem to a degree, by screening out wood fibre and using low or medium intensity magnetic separators for removing ferromagnetics. This has not, however, provided a complete solution.
It must be mentioned that some prior art separators have been made wherein this problem is automatically avoided as a result of the manner in which the magnetic materials are recovered. One such machine is described in U.S. Pat. No. 3,994,801 to Colburn in which a conveyor arrangement is provided and the whole mass of matrix is demagnetised and allowed to tumble freely during washing thereof to recover the magnetic materials. Applicants consider this apparatus to be too complicated or costly, or both, for many applications where a simple annular housing, rotatable about a horizontal axis, is considered desirable.
Another apparatus where the entire matrix is tumbled and washed to recover the magnetic material is described in the Soviet Journal of Non-Ferrous Metals Vo. 10, No. 9, September 1969, at page 35. In this case, an annular framework is rotatable about a horizontal axis and the compartments holding the matrix are all completely emptied to recover the magnetics. A major drawback of removing the entire matrix is the power required to handle the matrix material.
There is thus no simple arrangement known to applicant wherein the magnetic materials can be recovered from the matrix without totally freeing all elements thereof and which yet provides for adequate cleaning of the matrix from contaminants automatically.